Removal of calcium and magnesium from rock-salt brine



rock salt'in water, hereinafter called brine.

added in such Patented Aug. 24, 1926.

1 UNITED STATES- PATENT- OFFICE.

FBLNOIS AI BTHUB IREETH, OF SANDIWLY, AND LESLIE ALEXANNEB IO'NBO, OI

NORTH'WICH,-IENGLAND, ASSIGNORS TO SOLVAYE NEW YORK, A CORPORATION 01 NEW YORK.

ROCESS COMPANY, OF BOLVAY,

Remover. or carom! annmemisluurnon nocxlsnr mama.

no Drawing. Application filed August 21 1920,

I This invention "relates to an improved process for removing calcium and magnesium from rock salt br ne or solutions of Itis well known that brine may be purified by'adding to it a mixture ofcarbonate of soda and hydrate of calcium, hereinafter called the reagents. The reagents are enters into double ecomposition and the sodium carbonate is equivalent to the cal cium present in the brine+the calcium added as a reagent. The following equation illustrates the reaction for the case in which the calcium and magnesiumordinarily present in the brine are in equivalent proportions. 7

When this'process is carried out in practice, it is found that the colloidal mixed I or precipitate settles extremely slowly so that the clear liquid can only be removed by decantation after the lapse of many hours. 7

Our invention depends on the observation that it is possible to transform the physical condition of this precipitate so that it as sumes'a form in which it settles very rapidly t In the process according to our invention therefore we first submit the liquid and precipitate to treatment during a short induction period in such manner as to obtain this peculiar transformation and we then rapidly separate the clear liquor by decantation. V

v The chief factors governing the transformationdare as follows-:-

(a) Excess of hydroxyl ion, as distinct I from excess of carbonate, is to be ri orously avoided. Hitherto it has been usua to add from '10-%"to100'% excess of-both reagents and as the precipitate does not dissolve in such excessyit would be expected that the excess would exert'a salting out action tending to coagulate the colloidal precipitate. We have found on'the contrary that the excess of OH ion should be as small as possible consistent with successful oper-.

ation of theprocess one large scale. The

amount of Ca(OH added 'must'therefore uantities that the lime is. equivalent to t e ma esium with which it Serial No. 405,169, and n emt 3mm February 15, 1919.

vbe very carefully controlled by analysis of thebrlne as often as required,

The effect of excess of OH ion is to ac celerate the induction period enormousl so that when. the recipitate is agitate it passes very quic 1y through the stage in which it can settle rapidly and then disintegrates when its power of J settling rapidly is-lost. This result is curious, since in many cases of colloidal solution or sus- 1 pension, the stability of the colloid increases as the solution approaches neutrality, whereas in this case the stability decreases as the liquid becomes less alkaline,

so that,theprecipi tate settles more rapidly. Whatever may be the theory, we have'observed the definite fact that excess of hydroxyl ion should be kept as low as possible.

(b) The liquid and precipitate must be agitated enough to .mix the reagents thoroughly with all parts of the liquid.

vBut we have also found that it is most important not to stir too long, since in that case the precipitate is partially idisintee grated into fine particles and it is'then impossible to obtain rapid and complete settling. A limited amount. of agitationi's therefore employed and in practice we prefer to stir gently throu-ghout'the induction period though in some cases we may stir violently for a brief partof the induction period (taking care to avoid disintegration) and then leave the mixture quiescent during the remainder of the induction period while the transformation is being completed.

(a) It is further necessary to avoid the presence of disintegrated precipitate from a Previous Operation and this is particularly affects the result. If the percentage of mag nesium is higher than usual, the precipitate will tolerate more stirring without disintegration. 1 Q A In general it is necessary to balance or correlate the abov'e'factors for each par checked by large scale experiments in view I liquid is adjusted with respect to the volof the difi'erence in the kind of agitation obtainable on the large scale. In all cases however the essential feature is to subject the brine and' precipitate to treatment during a short induction period in which the precipitate is transformed into a different physical condition, and to avoid stringently any treatment which causes disintegration.

here is an optimum treatment which must not be prolonged too far. After treatment the clear liquor is then decanted rapidly.

Example, To a brine containing calcium and magnesium expressed as 0.055 grammes MgO and 0.252 grammes CaO per 100 cubic centimetres maintained at a temperature of about 25 and stirred with a stirrer having a peripheral speed of about 40 feet per mlnute and of such design that an even gentle motion is imparted to all portions of'the liquid, there is added, 0.63 grammes.

Na GO and 0.102 grammes (Ja(OH) per 100 cubic centimetres of brine. After stirring for halfan hour the precipitate subsides rapidly and can be settled by any well known means; the clear brine is free from calcium and magnesium.

The process may be conducted either intermittentl in which case the reagents may if desired be added before stirring begins, or continuously, in which case the stirring is preferably continued all the time; in continuous operation the rate of flow of ume of the containing vessel so that the particles are exposed to agitation for about half an hour.

We declare that what we claim is 1. The process of effectively freeing brine from calcium and magnesium impurities,

which consists in first determining the amount of said impurities contained in 'the bfi'ine and then addingv to the brine lime to t e the ormation and precipitation of the magnesium with but a very slight, insignificant excess of lime, insuflicient to cause material retardation in the settling of the precipitate fprecise extent indicated by. theory for over the settling rate which obtains when brine and then adding to the brine lime to the precise extent indicated bytheory for the formation and precipitation of the magnesium with but a very slight, insignificant excess I of lime, insufficient to cause material retardation in the settling of the precipitate over the settling rate which obtains when the relation of magnesium expressed as MgO to the lime as Ca(OH) is as .055 :.102, adding to the brine also sodium carbonate in an amount. sufiicient to satisfy that needed for the formation of precipitatable calcium salt from both the calcium originally present in the brine and that added as lime and subjecting the admixture of brine and precipitates to limited agitation, insuflicient to cause disintegration of the precipitated particles, but sufficient to allow a change in the physical condition of the precipitate tb take place in the direction of increasing the rate of sedimentation thereof.

3. A process as claimed in claim 1 in which the liquid and precipitate are 'maintained at a temperature of about 25 C.

In Witness whereof, we have hereunto signedour names this 5 ,day of August 1920.

FRANCIS ARTHUR FREETH. LESLIE ALEXANDER MUNRO. 

